Bright corrosion resistant coating of metals



according to the present Patented Feb. 17, 195 3 BRIGHT CORROSIONRESISTANT COATING METALS Charles W. Ostrander, Baltimore, Md., assignorto 'Rheem Manufacturing Company, New York,

N. Y., a corporation of Californiav No Drawing. Application June 24,1947,

Serial No. 756,813

7 Claims. (01. 148-621) The present invention relates to the treatmentof metal surfaces containing zinc, cadmium, aluminum, copper andalloysincluding appreciable amounts thereof. and is useful both in'connection with castings and coatings of these metals, notablygalvanized iron.

The principal object of the invention is to impart a bright,colorless,water-repellant surface or film, having improved corrisionresistance, to zinc, cadmium, aluminum and copper, and their alloys, bysubjecting the metal surface to a single immersion treatment. M Anotherobject of the invention is to provide a treating solution and methodwhich will enable the above-mentioned results to be uniformly obtainedon a commercial scale.

A further object of the invention is to provide a treating solution andmethod which willrender the metal surface suitable for the reception ofpaint. That is, whereas the untreated metal surfaces will not affordadequate paint adhesion for commercial purposes, metal surfaces treatedinvention exhibit marked paint adherence.

The use of solutions containing chromic acid and sulphuric acid toimpart corrosion resistance usually causes discoloration. I have foundthat a clear coating can be obtained by including acetic acid in suchsolutions and maintaining a relatively high acidity. In any case, wherethe acid number of the solution is relatively low and the acetic acid isnot sufficiently active, any tendency toward discoloration is overcomeby the addition of nitric acid.

I The term acid number of the solution as used herein is' based upon theresults obtained by conventional acidimetry practice where it is usualfor the determination of the amount of acid present in solutions to basecalculations.

upon the amount of standardized solution of alkali required by titrationto neutralize the acid.

In explanation, a solution of chromic acid (C103) and sulphuric acid inthe ratio of 16 grams of chromic acid per liter to 1 cubic centimeterper liter of sulphuric acid will produce discoloration. However, whenacetic acid is included in such a solution and the acid number thereofis between about 150 and 250, discoloration is avoided. When for anyreason, the acid number of the solution is below about 150, or theacetic acid 'is not sufllciently effective to preelude discoloration byreason of its inactivity with respect to the metal surface, thissituation is readily avoided by the addition of nitric acid.

The results described are particularly true in the case of zinc, alloysof zinc and galvamzed "coatings. v v V Treatments of the characterdescribed have certain practical requirements, notably economy in theuse of the chemicals making up the solution, and the speed and facilitywith which the desired results may be obtained. With the presentinvention, an appreciably lower ratio of chromic acid and sulphuric acidhas been found satisfactory, namely 16:1, and of equal importance, asingle dip in the solution produces the improved colorless film.

The invention is operative through a wide range of acidities of thesolutions employed, as illustrated in the following examples:

Example I Chromic acid, gms./l 160 Sulphuric acid cc./l l0 Acetic acidcc./l 80 Solution acid No 200.0

m I Example 1H Chromic acid grams/liter H20" 160 Acetic acid cc./literH20" 99 vSulphuric acid cc./liter H20 10 Example III Chromic acid c-gms./l 80 Sulphuric acid cc./l 5 Acetic acid cc./l Nitric acid cc./l 10Solution acid No 112.?-

Example IV Chromic acid gms./l 40 Sulphuric acid cc./l 2.5 Acetic acidcc./l Nitric acid cc./l 10 Solution acid No 96.8

I Example V Chromic acid --gms./l 20 Sulphuric acid cc./l 1.25 Aceticvacid cc./l- 80 Nitric. acid cc./l 10 Solution acid No 79.0

i I Example VI' Chromic acid gms./l l0 Sulphuric acid cc./l 0.625 Aceticacid cc./l 80 Nitric acid cc./l 10 Solution acid No 70.5

literof water. 1

In preparing the mixes of the foregoing examples, the chemicals aresupplied in concentrated form, the concentration being to the practicallimits of solubility. As will be appreciated, this affords a substantialsaving in shipping since the amount of water will be much less than thatindicated above for the actual Working solutions.

For example, 12 gallons of concentrate according to Example I is placedin a carboy and will consist of 66.8 pounds of chromic acid (Ci-Os), 40pounds acetic acid (99.6% acetic acid), sulphuric acid 1899.6 cc. andwater to make 12 gallons. Such concentrate, when used, will be dilutedwith three parts of water so as to produce a bath of 50 gallons.

In the case of Example III, the ingredients are placed in a carboy inthe amount of 12 /2 gallons. This concentrate will consist of chromicacid 50 pounds, acetic acid 4.12 gallons (99.5 acetic acid),

sulfuric acid 1453.8 cc., nitric acid 2635 cc. and

sufficient water to make 12 gallons. This concentrate is prepared forworking operations by introducing parts of water to make a bathcontaining 75 gallons.

The use of aceticacid is necessary in all of the mixtures to produce abright, colorless, film by a single treatment or immersion. The amountof acetic acid recited is slightly in excess of that theoreticallyrequired in order to take care of vaporization occasioned by the heatingof the solution during treatment.

Although ratios of chromic acid to sulphuric acid of from 1:12 to 1130may be used in this invention, from the practical standpoint, ratiosbelow about 1:16 or above about 1:16 present objections from thestandpoint of commercial operation in many cases.

It will be observed that Example I possesses an aciditysufficiently'highfor the acetic acid alone to produce a colorless,bright film. On the other 1 hand, the acidity of solutions III to VI isrelatively lower, and the acidity of these formulations is renderedeffective by means of a strong inexpensive acid which, insolution withchromic acid, will not produce a colored film. Nitric acid possessesthis characteristic in that even minute additions of sulphuric andhydrochloric acid to the compositions of the foregoing examples willresult in discoloration. The nitric acid appears to stabilize thesolution, or in other words, where the activity of theacetic acid aloneis insuflicient to prevent discoloration, as where the acid number ofthe solution istoo low, the addition of the nitric acid accelerates andsupplements the action of the acetic acid.

If chromic acid is not available, sodium bichromate can be substitutedfor part of the chromic acid, and excess nitric acid may be employed insuch a case to maintain the desired acidity, if necessary.

For carrying out the treatment, the solution may be maintained at atemperature from 60 to 212 F. and the period of immersion may extendfrom 1 to 120 seconds, depending upon concentration, acid number andtemperature of the solution. A preferable temperature range for thesolution is between 85 and 95 F. and a preferable immersion period isbetween 5.0 and econds.

In the examples, the amount of chromic acid may be increased to as muchas 320 grams provided the ratio of sulphuric acid to chromic acid ismaintained at about 1:16;. The amount of acetic. acid. included in thesolution may vary from to 200 cc, per liter and. the;amcunt of nitricacid from 1 to 50 cc. per liter. The examples above-recited are,preferred, and have particular value for the treatment of zinc, zincalloys and galvanized metal.

In some cases it is desirable to introduce a suitable wetting agent inthe bath in order to overcome any tendency of the coating to develop ablue or yellow streak on fiat work.

After the materials have been treated in the bath, a hot water rinse ispreferably employed to remove any streaking.

In addition to imparting corrosion resistance, it is found that thecoating exhibits to a most surprising extent the desirable property ofimparting to the treated surface the characteristic of beingwater-repellent, e. g., on zinc.

So-called bright finishing dips are not necessary, but in some cases dueto the character of the zinc, cadmium, aluminum or copper surface,

an aqueous bright dip or rinse consisting of about one-half of one percent nitric acid or a dilute solution of caustic soda is used after thetreatment and accelerates the bringing out of a bright finish on thetreated article.

While I have indicated that a hot rinse is preferred after treatment,satisfactory results are obtained by a rinse in cold water.

It is preferred to immerse the castings of zinc, aluminum, cadmium, orcopper, or the articles coated with such metals in a bath, but thetreatment may, in some cases, be carried out by other means, as, forexample, by spraying the aqueous acidic solution upon the surfaces tobetreated.

The zinc, cadmium, aluminum and copper surfaces to be treated areintroduced into the aqueous bath until a preferred, bright surface isformed. It is desirable to agitate the bath during the treatment.Articles treated with the solution in the manner above set forth, as byimmersion or spraying, and particularly zinc, exhibit an enhanced saltspray resistance.

A significant feature of the invention, as aboveindicated, is themaintenance of a critical acidity. That is, when the acid number of thesolution is above 150, the acetic acid alone is active to preventdiscoloration. Where, however, the acid number of the solution is below150, it is necessary to include nitric acid in order to avoiddiscoloration. It is found that solutions having an acid number fromabout 50 to- 250 are preferable, the acetic acid being effective alonewhen the acid number is between and 250, and the nitric acid being addedwhen the acid number of the solution isbelow 150 to produce a solutionhaving an acid number between 50 and 150.

In the appended claims, the expression zinc, cadmium, aluminum or copperarticles is intended to cover castings and coatings of zinc, cadmium,aluminum, or copper, or alloys thereof, as Welles mixtures in which thealuminum, zinc, cadmium or copper are present in substantial amount.

This application is a continuation-in-part of my application Serial No.648,823, filed February 19, 1946, now abandoned.

I claim:

1. An aqueous acidic solution for treating metals selected from thegroup consisting of zinc and cadmium and alloys thereof to impart abright, colorless surface to the metal consisting essentlally of chromicacid 10 grams per liter to 320 grams per liter, sulfuric acid and aceticacid, the chromic acid and sulfuric acid being present in the ratio ofabout 1 cc. per liter of sulfuric acid to about 16 grams per liter ofchromic acid, the

acetic acid being present-in amount of about 25 to about 200 cc.-perliter.

An aqueous acidic solution according to claim 1 containing nitric acidin amount of about 1 to about 50 cc. per liter.

3. A process of treating metals of the group consisting of zinc andcadmium and alloys thereof which comprises subjecting the same to anaqueous acidic solution consisting essentially of chromic acid grams perliter to 320 grams per liter, sulfuric acid and acetic acid, the chromicacid and sulfuric acid being present in the ratio of about 1 cc. perliter of sulfuric acid to about 16 grams per liter of chromic acid, theacetic acid being present in amount or about to about 200 cc. per liter,the temperature of the solution being maintained between about 60 and212 F. and the treatment being continued until a bright, colorlesssurface is imparted to the metal, removing the metal from the bath andrinsing the metal.

4. The process according to claim 3 wherein the solution contains nitricacid in amount of about 1 to about cc. per liter.

5. A process of coating zinc which comprises immersing the zinc in asolution consisting of about 48 to 288 grams per liter chromic acid,about 5.1 to 34.2 grams per liter H2SO4, about 2 to 41 grams per literHNOa, about 26.3 to 82 grams per liter HCzHzOz, and the balance water atF. to F., the treatment being continued until a bright colorlesstransparent protective coating is imparted to the zinc, and removing thezinc from the solution.

6. A process of coating zinc which comprises immersing the zinc in asolution consisting of about 10 to 320 grams per liter chromic acid,about 1.4 to 34.2 grams per liter H2804, about 1.4 to 70.0 grams perliter HNO3, about 26.0 to 2081) grams per liter HCzHsOz, and the balancewater at 60 F. to 212 F., the treatment being continued until a brightcolorlesstransparent protective coating is imparted to the zinc, andremoving the zinc from the solution.

7. An aqueous acidic solution for treating zinc to impart a brightcolorless transparent protective coating thereto consisting of about 10to 320 grams per liter chromic acid, about 1.4 to 34.2 grams per literH2804, about 1.4 to 70.0 grams per liter HNOs, about 26.0 to 208.0 gramsper liter HC2H302, and the balance water.

CHARLES W. OSTRANDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number 7 Name Date 2,021,592 Dubpernell et a1.Nov. 19, 1935 2,224,245 Allen Dec. 10, 1940 2,376,158 McCarroll et a1May 15, 1945 2,433,723 Wieczorek Dec. 30, 1947 FOREIGN PATENTS NumberCountry Date 558,983 Great Britain Jan. 31, 1944

1. AN AQUEOUS ACIDIC SOLUTION FOR TREATING METALS SELECTED FROM THEGROUP CONSISTING OF ZINC AND CADMIUM AND ALLOYS THEREOF TO IMPART ABRIGHT COLORLESS SURFACE TO THE METAL CONSISTING ESSENTIALLY OF CHROMICACID 10 GRAMS PER LITER TO 320 GRAMS PER LITER, SULFURIC ACID AND ACETICACID, THE CHROMIC ACID AND SULFURIC ACID BEING PRESENT IN THE RATIO OFABOUT 1 CC. PER LITER OF SULFURIC ACID TO ABOUT 16 GRAMS PER LITER OFCHROMIC ACID, THE ACETIC ACID BEING PRESENT IN AMOUNT OF ABOUT 25 TOABOUT 200 CC. PR LITER.